1. Field of the Invention
This invention relates to antennas for transmitting or receiving electromagnetic energy. More specifically, this invention relates to millimeter and submillimeter wave antennas.
While the present invention is described herein with reference to a particular embodiment for a particular application, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
Conventional imaging systems which utilize infrared or visible light typically provide images of superior resolution under favorable atmospheric conditions. As is well known, however, environments laden with smoke, smog or fog may impede propagation of infrared or visible light thereby obscuring a scene to be imaged. Imaging systems designed to be operative under such adverse environmental conditions have tended to rely on lower frequency electromagnetic radiation. For example microwave imaging systems more effectively penetrate fog and smoke than do those using infrared or visible light. However, systems utilizing longer wavelength microwave radiation typically generate images having less resolution than images produced by higher frequency systems.
Millimeter and submillimeter wave imaging systems offer improved resolution relative to microwave systems while still exhibiting good fog and smoke penetration capability. Conventional millimeter wave imaging systems have generally been comprised of either waveguide components or of detection components mounted on a dielectric substrate. Waveguide receiving antennas included in waveguide imaging systems are capable of generating well defined antenna patterns which may enhance image clarity. However, the small dimensions of millimeter and submillimeter waveguide imaging systems may significantly increase the cost of such systems. Milling tolerances on the order of microns and typically small detection elements are two examples of attributes of many millimeter and submillimeter waveguide detection systems which may contribute to their characteristically high cost. Further, millimeter and submillimeter waveguide antenna arrays have proven to be prohibitively expensive for numerous applications because of the large cost of each antenna element.
In single antenna imaging systems the antenna element scans regions of a scene to provide a composite image. While this method may render accurate images when used in applications such as radio astronomy where imaging speed is not of primary concern, this scanning process inherently slows image formation which makes single element systems inappropriate for certain applications. Alternatively, antenna arrays generally increase imaging speed as each antenna element is responsible for detecting a specified region of a scene to be imaged. Given the expense of fabricating millimeter and submillimeter waveguide antenna arrays, attempts have been made at developing arrays of antenna elements mounted on dielectric substrates. The substrates provide mechanical support for antenna elements typically having dimensions on the order of half a millimeter and often lacking structural rigidity. Additionally, well developed lithographic techniques can be borrowed from VLSI circuit technology to facilitate fabrication of antenna elements and their associated detection and signal processing components.
While substrate mounted imaging antenna arrays may be manufactured at a fraction of the cost of comparable millimeter waveguide antenna imaging arrays, substrate mounting presents numerous disadvantages. Electromagnetic patterns generated by antennas mounted on substrates tend to be inferior to those produced by antennas radiating in free space. Further, substrate mounted arrays generally have more losses and less power handling capability than comparable waveguide systems. In planar substrate mounted antenna arrays antenna elements and interconnections are fabricated on a common surface. This planar implementation generally involves at least two design tradeoffs. First, space devoted to interconnections cannot typically be utilized by antenna elements hence limiting the efficiency of collection of incident electromagnetic energy. Second, planar systems affording increased collection efficieny through a more dense concentration of antenna elements may experience performance degradation due to electromagnetic coupling between antenna elements.
Multi-layer substrate antenna arrays have attempted to improve collection efficiency by providing a separate substrate for interconnections. However, this multi-layer approach does not address the problem of parasitic coupling between antenna elements. Moreover, the orientation of the component substrates in the multi-layer implementation often requires holes to be fabricated through the substrates providing for interconnection. This process may be difficult and expensive as a result of the inherently small dimensions of millimeter and submillimeter imaging antenna arrays. Further, multi-layer structures generally cannot exploit existing low cost integrated circuit manufacturing processes available for planar, monolithic implementations.
Hence, a need in the art exists for an inexpensive two-dimensional millimeter and submillimeter wave substrate antenna array providing efficient collection of incident electromagnetic energy and having antenna elements relatively unimpaired by a mounting arrangement.